Format: support inline namespaces

[compnerd]

Summary

Correct handling for C++17 inline namespaces. We would previously fail to
identify the inline namespaces as a namespace name since multiple ones may be
concatenated now with C++17.

[djasper]

Expression can be simplified, otherwise looks good.

lib/Format/UnwrappedLineParser.cpp

1370

while (FormatTok->isOneOf(tok::identifier, tok::coloncolon))
  nextToken();

[compnerd]

SVN r251690 with the mod.

lib/Format/UnwrappedLineParser.cpp

     addUnwrappedLine();
 }
 
 void UnwrappedLineParser::parseNamespace() {
   assert(FormatTok->Tok.is(tok::kw_namespace) && "'namespace' expected");
 
   const FormatToken &InitialToken = *FormatTok;
   nextToken();
-  if (FormatTok->Tok.is(tok::identifier))
+  while (FormatTok->Tok.is(tok::identifier) ||
+         FormatTok->Tok.is(tok::coloncolon))
     nextToken();
   if (FormatTok->Tok.is(tok::l_brace)) {
     if (ShouldBreakBeforeBrace(Style, InitialToken))
       addUnwrappedLine();
 
     bool AddLevel = Style.NamespaceIndentation == FormatStyle::NI_All ||
                     (Style.NamespaceIndentation == FormatStyle::NI_Inner &&
                      DeclarationScopeStack.size() > 1);

unittests/Format/FormatTest.cpp

             "#endif // HEADER_GUARD",
             format("#ifndef HEADER_GUARD\n"
                    " #define HEADER_GUARD\n"
                    "   namespace my_namespace {\n"
                    "int i;\n"
                    "}    // my_namespace\n"
                    "#endif    // HEADER_GUARD"));
 
+  EXPECT_EQ("namespace A::B {\n"
+            "class C {};\n"
+            "}",
+            format("namespace A::B {\n"
+                   "class C {};\n"
+                   "}"));
+
   FormatStyle Style = getLLVMStyle();
   Style.NamespaceIndentation = FormatStyle::NI_All;
   EXPECT_EQ("namespace out {\n"
             "  int i;\n"
             "  namespace in {\n"
             "    int i;\n"
             "  } // namespace\n"
             "} // namespace",